This invention relates to instruments that measure evaporation of water from an open container, and that are used to determine the timeliness and appropriate amount of irrigation for plants and lawns. Such devices are commonly called pan evaporimeters.
Efficient irrigation starts with knowledge of the amount of water that has evaporated from an irrigated area since the previous irrigation. This amount is called evapotranspiration, or simply ET, and is measured in units of water height (inches or millimeters).
The rate of evapotranspiration from healthy, well-watered grass (inches/day or inches/week) is called "reference evapotranspiration." This rate is affected by solar radiation, humidity, temperature, and wind. As such, reference evapotranspiration provides an effective and quantitative measure of the weather for irrigation purposes.
Agriculturalists have long used reference evapotranspiration, combined with rainfall measurement, as the basis for scheduling the irrigation of pastures and crops (Cuenca, R. H., Irrigation System Design: An Engineering Approach, Prentice-Hall, Inc., Englewood Cliffs, N.J.; pp. 115-188 (1989)). However, with the increasing and widespread need to conserve water, ET information is now commonly used to schedule the irrigation of golf courses, parks, and city landscapes.
In some regions, homeowners are being advised of current daily or weekly ET rates through local newspapers and radio. The objective is to help people determine how long to run their lawn and garden sprinklers by knowing current evaporative conditions for their area.
Historically, the U.S. Weather Bureau Class "A" pan evaporimeter has been the most widely-used device to provide regional measurements of evapotranspiration throughout the world. These large devices are 48 inches in diameter, and are replenished daily to maintain a precise water depth of 8 inches. They are typically located at agricultural research facilities.
A predictable relationship between evaporation from U.S. Class "A" pans and evapotranspiration from well-watered grass has been established (Doorenbos, J. and Pruitt, W. O., "Crop Water Requirements," Food and Agriculture Organization of the United Nations, Irrigation and Drainage Paper No. 24, Rome, Italy (1977)). Such relationships exist because evaporation of water from pans, like that from plants, is responsive to radiation, wind, temperature, and humidity. Pans with a large water capacity, such as the Class "A" pan, however, exhibit heat storage that causes the evaporation rate to lag significantly behind radiation input. This can cause substantial evaporation at night when plants are not transpiring. Readings from large pans, therefore, are not particularly meaningful on a daily basis, but become more useful when averaged over a period of 7 to 10 days. Such intervals are appropriate for many agricultural applications, but not so for situations that require frequent water applications, such as the maintenance of high quality turfgrass.
Another disadvantage of large unscreened pans is that an open water surface absorbs more solar radiation than a grass surface (Thom etal., "On the Proper Employment of Evaporation Pans and Atometers in Estimating Potential Transpiration," Quarterly Journal of the Royal Meteorological Society, 107:711-736 (1981)). This contributes to the 25% to 33% greater evaporation from Class "A" pans surrounded by well-watered grass, than from the grass itself. Hence the need for pan coefficients of the order 0.75 to 0.85 (see Equation (1)). If a pan is not surrounded by green grass, and is subject to hot, drying winds, the pan coefficient can be as low as 0.35 (Doorenbos and Pruitt).
More recently, there has been a trend toward the use of automated meteorological stations to provide estimates of current ET. In California, USA, for example, more than ninety automated meteorological stations have been installed throughout the state to provide such information. These provide hourly measurements of solar radiation, wind speed, temperature, humidity, and rainfall for estimating daily evapotranspiration and effectiveness of rainfall. The devices are highly technical, expensive, and rely upon computers.
The above approaches, however, are not particularly practical for providing homeowners with information on current watering requirements. Furthermore, they do not adequately account for the effect of localized micro-climates and rainfall upon watering needs. Thus it can be appreciated there exists a need for a simple and inexpensive evaporimeter-type device that provides homeowners with a measure of the current irrigation requirement for their own particular location.
Ideally, the device would visually and readily indicate the net amount of water required to replace that which had evaporated since previous irrigation. That is, the difference between accumulative evapotranspiration loss and effective rainfall received, or "net evapotranspiration." To provide accurate daily readings, the device would preferably contain only a small volume of water so as to respond rapidly to radiation and temperature changes, be suited to placement close to a vegetative surface such as grass, and be of such design that the relationship between the evaporation rate of its water content and that from well-watered grass is known. That is, the device would be designed to have a known pan coefficient (K.sub.pan, Equation (1)). Furthermore, it would be an advantage if the device could be used to determine the application rate of sprinklers, and therefore the duration of watering required to replace net evapotranspiration losses.
A miniature and novel pan evaporimeter that achieves these and other objectives is the subject of this disclosure.